How to make SQLite preallocate memory during startup? - c

Question

I'm working on an embedded program that uses SQLite, and I want more predictable memory usage.
I read https://www.sqlite.org/malloc.html and wanted to use the "Zero-malloc memory allocator" described in section 3.1.4. If I understand it correctly, it tells SQLite to allocate a fixed amount of memory at startup, and use it exclusively, instead of making dynamic allocations (malloc) at runtime.
On my Ubuntu desktop (so nothing to do with embedded), I compiled the latest SQLite with -DSQLITE_ENABLE_MEMSYS5, created an example program which tells SQLite to preallocate 1GB RAM and then sleep for 15 seconds. I wrote a test script to check 'free' before and after running the example.
Unfortunately, I don't see the preallocation reflected in the 'free' output. What gives? My example checks every SQLite return code and I didn't get any errors. How can I make SQLite reserve the memory immediately during startup?
My test script:
#!/bin/bash
set -o verbose
free -b
./testsqlite3 &
sleep 3
ps aux | grep testsqlite3
free -b
Output:
$ ./runtest.sh
free -b
total used free shared buff/cache available
Mem: 8421400576 607322112 6244298752 28102656 1569779712 7066603520
Swap: 0 0 0
./testsqlite3 &
sleep 3
Using libsqlite version 3.40.0, sqlite3.h is version 3.40.0
Preallocating using MEMSYS5
Opened database successfully
Table created successfully
Sleeping for 15 secs before exiting. Call 'free -b' now.
echo "bash sleep complete, confirming app still running"
bash sleep complete, confirming app still running
ps aux | grep testsqlite3
user1 7474 0.0 0.0 979960 2380 pts/1 S+ 12:11 0:00 ./testsqlite3
user1 7477 0.0 0.0 5116 880 pts/1 S+ 12:11 0:00 grep testsqlite3
free -b
total used free shared buff/cache available
Mem: 8421400576 607420416 6244200448 28102656 1569779712 7066505216
Swap: 0 0 0
My example program testsqlite3:
#include <stdio.h>
#include <stdlib.h>
#include <sqlite3.h>
#include <unistd.h>
static int callback(void *NotUsed, int argc, char **argv, char **azColName) {
int i;
for(i = 0; i<argc; i++) {
printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL");
}
printf("\n");
return 0;
}
int main(int argc, char* argv[]) {
sqlite3 *db;
char *zErrMsg = 0;
int rc;
char *sql;
fprintf(stdout, "Using libsqlite version %s, sqlite3.h is version %s\n", sqlite3_libversion(), SQLITE_VERSION);
fprintf(stdout, "Preallocating using MEMSYS5\n");
//int preallocateMemoryBytes = 100000000; //100MB
int preallocateMemoryBytes = 1000000000; //1GB
rc = sqlite3_config(SQLITE_CONFIG_HEAP,
malloc(preallocateMemoryBytes),
preallocateMemoryBytes, 32);
if (rc != SQLITE_OK) {
fprintf(stderr, "failed to config SQLITE_CONFIG_HEAP\n");
return(-1);
}
rc = sqlite3_initialize();
if (rc != SQLITE_OK) {
fprintf(stderr, "initialize() failed\n");
return(-1);
}
//open DB
rc = sqlite3_open("test.db", &db);
if( rc ) {
fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
return(-1);
} else {
fprintf(stdout, "Opened database successfully\n");
}
sql = "CREATE TABLE IF NOT EXISTS COMPANY(" \
"ID INT PRIMARY KEY NOT NULL," \
"NAME TEXT NOT NULL," \
"AGE INT NOT NULL," \
"ADDRESS CHAR(50)," \
"SALARY REAL );";
rc = sqlite3_exec(db, sql, callback, 0, &zErrMsg);
if( rc != SQLITE_OK ){
fprintf(stderr, "SQL error: %s\n", zErrMsg);
sqlite3_free(zErrMsg);
return(-1);
} else {
fprintf(stdout, "Table created successfully\n");
}
fprintf(stdout, "Sleeping for 15 secs before exiting. Call 'free -b' now.\n");
usleep(15000 * 1000);
sqlite3_close(db);
return 0;
}